"There have been a lot on conflicting studies," said Kerry Key, a
seismologist at the Scripps Institution of Oceanography in San
Diego.

Understanding the nature of the boundary layer and its role in
plate tectonics is one of the grand challenges in seismology,
according a list assembled by the Incorporated Research
Institutions for Seismology in 2009.

Now, a new study co-authored by Key appears to confirm the
boundary zone is molten magma, at least under the ocean floor.
Off the coast of Nicaragua, beneath the Cocos tectonic plate,
researchers discovered a 15-mile-thick (25 kilometers) layer of
partially melted rock at the bottom of the lithosphere. The
results are published today (March 20) in the journal Nature.

"It's really a surprise," said Samer Naif, a Scripps graduate
student and lead author of the study."We went out to try and
understand the crustal fluid cycle at a subduction zone and we
stumbled upon a partial-melt layer."

In past decades, the dominant view was that the boundary layer
was likely melt-free, but weakened by water-rich minerals, Naif
said. But in the last five years, new studies based on earthquake
waves passing through the layer suggested the zone was
molten, at least in certain places, he said.

The researchers saw the molten zone beneath the Cocos plate while
using a technique that looks for subtle variations in Earth's
naturally occurring electric and magnetic fields. These
variations reveal structures below the surface, and are
particularly effective at revealing pockets of liquid, such as
oil and gas reservoirs.

"We've come out of left field with electromagnetic data, which
shows much more sensitivity to features like this," Naif said. We
could potentially have a lot more to say [about the boundary
layer] if we go out and do more surveys," he told
OurAmazingPlanet.

The
lithosphere-asthenosphere boundary (LAB) puts the "plate" in
plate tectonics, marking the bottom of the stiff tectonic plates
that shift on Earth's surface, riding convection currents deeper
in Earth's mantle. Both layers are solid rock, but the
lithosphere is hard, stiff and cold and the hotter asthenosphere
flows and deforms on a geologic time scale. The discontinuity,
molten or not, lies at depths from 30 miles (50 km) under the
ocean floor to 120 miles (200 km) beneath continents.

The group's next step is to explain why the magma is there, Key
said. Other studies suggest that older ocean lithosphere does not
have a molten LAB, Naif added. The geologically young Cocos plate
could have remnant magma plastered to its underbelly from its
birth at a nearby mid-ocean spreading ridge, for example.